A valve is a device used to control fluid flow, pressure, and direction. The controlled fluid can be liquid, gas, gas-liquid mixture, or solid-liquid mixture.

Valves are usually composed of valve bodies, valve covers, valve seats, opening and closing components, driving mechanisms, seals, and fasteners. The control function of a valve is achieved by driving a mechanism or fluid to drive the lifting, sliding, swinging, or rotary motion of the opening and closing parts to change the size of the flow channel area.

Valves have a wide range of uses and are closely related to people"s daily lives, such as faucets for water pipes and pressure reducing valves for liquefied petroleum gas stoves. Valves are also indispensable components in various mechanical equipment such as internal combustion engines, steam engine, compressors, pumps, pneumatic transmission devices, hydraulic transmission devices, vehicles, ships and aircraft.

Two thousand years ago, Chinese people used bamboo pipes and wooden plug valves on water pipelines, and later used water gates on irrigation channels, plate check valves on smelting bellows, and bamboo pipes and plate check valves to extract salt water in well salt mining.

With the development of smelting technology and hydraulic machinery, copper and lead plug valves have emerged in Europe. With the use of boilers, lever heavy hammer safety valves appeared again in 1681. Before the advent of Watt steam engine in 1769, plug valves and check valves were always the most important valves.

The invention of the steam engine brought valves into the field of mechanical industry. In addition to plug valves, safety valves and check valves, Watt"s steam engine also uses butterfly valves to regulate flow. With the increase of steam flow and pressure, the use of plug valves to control the inlet and exhaust of the steam engine can no longer meet the needs, resulting in the emergence of slide valves.

Around 1840, there were successively globe valves with threaded stems and wedge gate valves with trapezoidal threaded stems, which was a major breakthrough in the development of valves. The emergence of these two types of valves not only met the requirements of various industries at that time for continuously increasing pressure and temperature, but also preliminarily met the requirements for flow regulation. Subsequently, with the development of the power industry, petroleum industry, chemical industry, and shipbuilding industry, various high and medium pressure valves developed rapidly.

After World War II, due to the development of polymer materials, lubricating materials, stainless steel, and cobalt based hard alloys, ancient plug valves and butterfly valves gained new applications, and ball and diaphragm valves rapidly developed. The variety and quality of globe valves, gate valves, and other valves have increased. The valve manufacturing industry has gradually become an important sector of the mechanical industry.

Valves can be divided into six categories based on their usage functions: block valves, regulating valves, check valves, diverter valves, safety valves, and multi-purpose valves.

Block valves are mainly used to block fluid passages, including globe valves, gate valves, plug valves, ball valves, butterfly valves, diaphragm valves, pinch valves, etc; Regulating valves are mainly used to regulate the pressure and flow of fluids, including regulating valves, throttle valves, pressure reducing valves, and floating ball regulating valves; Check valves are used to prevent reverse flow of fluid; Diverter valves are used to distribute the direction of the flow path or separate two-phase fluids, including slide valves, multi way valves, drain valves, and exhaust valves; Safety valves are mainly used for safety protection to prevent damage to boilers, pressure vessels, or pipelines due to overpressure; A multi-purpose valve is a valve that has more than one function, such as a stop check valve, which can act as both a shut-off valve and a check valve.

Industrial pipeline valves can be divided into vacuum valves, low-pressure valves, medium pressure valves, high-pressure valves, and ultra-high pressure valves according to their nominal pressure; Valves can be divided into room temperature valves, medium temperature valves, high-temperature valves, and low-temperature valves according to their working temperature; Valves can also be classified according to the type of driving device, the connection method with the pipeline, and the material used for the valve body. Valves can be named individually or in combination according to various classification methods, as well as according to the structural characteristics or specific purposes of the opening and closing components.

The basic parameters of a valve are working pressure, working temperature, and diameter. For various valves widely used in industrial pipelines, nominal pressure and nominal diameter are commonly used as basic parameters. The nominal pressure refers to the maximum allowable working pressure that a valve of a certain material can withstand at a specified temperature. The nominal diameter refers to the nominal inner diameter of the connecting end between the valve body and the pipe.

Valves have different requirements based on their type and purpose, mainly including sealing, strength, regulation, circulation, opening and closing performance. When designing and selecting valves, in addition to basic parameters and performance, the performance of the fluid should also be considered, including its phase state (gas, liquid or containing solid particles), corrosiveness, viscosity, toxicity, flammability, explosiveness, rarity, and radioactivity.

Sealing performance and strength performance are the most basic and important performance of all valves. The sealing of valves is divided into two parts: internal sealing and external sealing. Internal sealing is the sealing between the valve disc and the valve seat; External sealing refers to the sealing between the moving parts of the valve stem and the valve cover, between the valve body and the valve cover, and between the connection parts of the valve body and the pipeline. When using valves, it is not only required to have good sealing performance, but also to ensure safety.

If leakage occurs due to poor sealing or parts are damaged due to insufficient strength, it will cause varying degrees of economic losses, such as transporting toxic, flammable, explosive, or highly corrosive fluids, and may also lead to serious safety accidents. In order to ensure the sealing and strength of the valve, in addition to complying with relevant standards and regulations for reasonable structural design and ensuring process quality, it is also necessary to select materials correctly.

Usually, valves for low pressure non corrosive fluids use cast iron or cast copper; Cast or forged steel for high and medium pressure valves; Alloy steel is used for high-temperature or high-pressure valves; Valves used for corrosive fluids are made of stainless steel, plastic, corrosion-resistant alloys (such as copper nickel molybdenum alloy, titanium alloy, and lead alloy), or corrosion-resistant materials lined with cast iron or cast steel.

Usually, the sealing surface of low-pressure valves is mostly made of brass or bronze, while high and medium pressure valves are mostly made of stainless steel. High demand high and medium pressure valves or high-temperature valves are made of cobalt based hard alloy. Polymer materials have been widely used in valves, such as the valve seat of ball valves mainly made of polytetrafluoroethylene plastic, and the sealing ring of butterfly valves and the diaphragm of diaphragm valves made of various rubber materials. These materials have better sealing properties than metals within the temperature range that can be used.

With the development of modern nuclear industry, petrochemical industry, electronic industry and space industry, as well as the development of automatic process control and long-distance fluid transmission, the development of modern cryogenic valves, vacuum valves, valves for nuclear industry and various control valves has been promoted. The application of valve drive devices for remote control and program control is increasing.

In the future, the development of valves will expand the product parameters, develop energy-saving, labor-saving and automatic control valves, improve the structure, adopt new materials and processes, improve the service life of valves, and develop special valve series, such as low-temperature valves, vacuum valves, valves for nuclear industry industry, safety valves, regulating valves, steam traps and valve actuators for liquid oxygen, liquid hydrogen and LNG.